The present invention is related to the method and apparatus described and claimed in U.S. Pat. Nos. 3,518,157 and 3,583,030, respectively, to Terry et al. More particularly, the present invention is directed at solving some problems which have resulted in one of the products manufactured using the aforesaid method and apparatus.
In each of the Terry et al patents, FIG. 10 depicts a fiber duct board in which the edge portions are compressed more than the remainder of the board. These edges, then, have a greater density and define a shoulder which provides a reference surface for overlapping or abutting multiple widths of the board to form a rectangular duct. The greater edge density strengthens the ultimate duct in the vicinity of the longitudinal joints.
In order to manipulate the fiber mats of the Terry et al. patents, it is necessary to partially cure the thermosetting binder in the vicinity of the two laterally extending surfaces. This makes the mat easier to handle by giving it definition and enables it to be further formed and shaped more easily. The difficulty arises specifically with the mat configuration which has the compressed edges. The partial cure causes the fibers on the surface to adhere to one another. When the edges of the partially cured mat are compressed more than the remainder, the adhering fibers are stretched. Once the compression is released, there is a tendency of these stretched fibers to spring back to a lesser compression. The shoulders on the fiber board become rounded as the fibers return to a position of lesser compression. In the assembled duct, these rounded shoulders produce line, rather than surface, contact at the joints. Such line contact can enable these joints to slip and can produce inadequate sealing resulting in leakage.
The present method and apparatus remedy this spring back problem. Following the initial curing of the laterally extending faces of the mat, longitudinal slits are formed adjacent the regions where the largest amounts of compression will occur. These slits are deep enough to penetrate through the partial cure on the surfaces of the mat to the uncured fibers beneath. The regions which are to be more compressed than the remainder of the mat can then be formed and shaped without suffering the tendency to spring back which was experienced in previous manufacturing efforts. These and other characteristics, features, and advantages of the present invention will become more apparent after a reading of the following descrptiion.